This application is based on application No. H11-077045 filed in Japan on Mar. 23, 1999, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image forming optical system that forms an image-carrying light beam by illuminating a reflective type spatial light modulator. The present invention relates also to an observation apparatus or projection apparatus employing such an image forming optical system.
2. Description of the Prior Art
In recent years, reflective type spatial light modulators such as reflective type liquid crystal devices have been receiving much attention. This is because their relatively high aperture ratios offer bright images and allow high-speed modulation. More specifically, on a liquid crystal device, circuits need to be formed to control the individual pixels thereof, and these circuits occupy a certain area on the liquid crystal device. Whereas, in a transmissive type liquid crystal device, the presence of those circuits inevitably reduces the aperture ratio, in a reflective type liquid crystal device, those circuits can be formed on the face of the device opposite to the face thereof from which an image-carrying light beam exits, and thus their presence does not reduce the aperture ratio. This permits reflective type liquid crystal devices to offer brighter images.
Accordingly, as compared with a transmissive type liquid crystal device having an identical number of pixels, a reflective type liquid crystal device can be made smaller without sacrificing brightness. To put it another way, as compared with a transmissive type liquid crystal device of an identical size, a reflective type liquid crystal device can be provided with more pixels so as to offer higher resolution. Thus, by the use of a reflective type spatial light modulator having these advantages, it is possible to form images with satisfactory brightness and resolution.
In a reflective type spatial light modulator, an image-carrying light beam exits from the same face of the device as that on which an illumination light beam is shone. Therefore, here, it is essential to separate the image-carrying light beam from the illumination light beam in some way. How these two light beams are separated in conventional arrangements will be described below, taking up three conventional image forming optical systems as Conventional Examples 1 to 3.
FIG. 7A schematically shows the overall construction of the image forming optical system of Conventional Example 1. In Conventional Example 1, a semitransparent surface 102 is disposed in front of a reflective type spatial light modulator 101. Here, the illumination light beam 103 (indicated by a solid line) is transmitted through the semitransparent surface 102 so as to be shone on the reflective type modulator 101, and the image-carrying light beam 104 (indicated by a dash-and-dot line) resulting from the illumination light beam being modulated thereby is reflected from the semitransparent surface 102 and is thereby separated from the illumination light beam 103. Alternatively, as shown in FIG. 7B, it is also possible to separate the illumination light beam 103 and the image-carrying light beam 104 by letting the semitransparent surface 102 reflect the former and transmit the latter.
FIG. 8 shows the overall construction of Conventional Example 2. In Conventional Example 2, a quarter-wave plate 109 is disposed in front of a reflective type modulator 101 to perform polarization conversion on the illumination light beam 106 and the image-carrying light beam 108, and a polarization selection surface 105 is used to separate the two light beams. Here, the illumination light beam 106 transmitted through the polarization selection surface 105 is, after being subjected to polarization conversion performed by the quarter-wave plate 109, reflected from the polarization selection surface 105. This helps minimize the loss of light. For example, in a case where the polarization selection surface 105 is so designed as to transmit only P-polarized light, only the P-polarized light component of the illumination light beam 106 is transmitted through the polarization selection surface 105 so as to be shone on the reflective type modulator 101, and the image-carrying light beam 108, which has been so converted as to include only S-polarized light, is reflected from the polarization selection surface 105 and is thereby separated from the illumination light beam 106.
In Conventional Example 3, one half of the pupil of a projection optical system is used by the illumination light beam and the other half thereof is used by the image-carrying light beam that is to be projected. This helps eliminate the loss of the illumination light beam. An optical system of this type is disclosed, for example, in Japanese Laid-Open Patent Application No. H9-96867.
However, in Conventional Example 1, before the image-carrying light beam is separated from the illumination light beam, it needs to have passed through the semitransparent surface twice. Thus, if it is assumed that the semitransparent surface transmits 50% of the light incident thereon, as little as 25%, i.e. our fourth, of the illumination light beam can be used as the image-carrying light beam. In particular, in a case where a liquid crystal device is used as the reflective type modulator, a light beam polarized in a particular direction needs to be shone thereon as the illumination light beam, and this aggravates the loss of light.
In Conventional Example 2, the loss of light is smaller than in Conventional Example 1. However, here, the polarization selection surface cannot be realized without the use of an optical element such as a PBS mirror or PBS block that exhibits incident-angle dependence. Specifically, the illumination light beam shone on the polarization selection surface includes rays that are incident thereon at various angles of incidence, and, of these rays, those whose angles of incidence have not been optimized hamper proper polarization selection and thus hamper proper separation of light to be transmitted and light to be reflected. This not only requires that the angle of incidence of the illumination light beam be controlled within a considerably narrow margin, but also leads to an unduly large loss of light or causes undesirable stray light to appear. Moreover, a PBS mirror, PBS block, or the like requires the formation of a PBS film, and is thus expensive. In particular, in a case where a high-precision PBS block is required, it needs to be formed by cementing a plurality of blocks together, and is thus troublesome and expensive to produce.
In Conventional Example 3, the optical system for projecting the image-carrying light beam needs to have a large pupil. This makes it difficult to secure satisfactory optical performance, and makes the optical system as a whole larger.
An object of the present invention is to provide a simple and inexpensive image forming optical system that is so designed as to waste as little light as possible and that is so constructed as to help the observation apparatus or projection apparatus into which it is incorporated to be made more compact.
To achieve the above object, according to one aspect of the present invention, an optical apparatus is provided with: a reflective type spatial light modulator for modulating an incoming light beam shone into the optical apparatus by selectively reflecting the incoming light beam; a light beam selecting surface for selectively reflecting the incoming light beam according to the angle of incidence at which the incoming light beam strikes the light beam selecting surface; and deflecting means disposed between the reflective type spatial light modulator and the light beam selecting surface so as to deflect at least one of the incoming light beam and the reflected light beam reflected from the reflective type spatial light modulator. Here, the incoming light beam is reflected from or transmitted through the light beam selecting surface so as to be shone, through the deflecting means, on the reflective type spatial light modulator, and is then selectively reflected by the reflective type spatial light modulator so as to be modulated into the reflected light beam, which is then shone, through the deflecting means again, on the light beam selecting surface so as to be transmitted therethrough or reflected therefrom according to whether the incoming light has been reflected therefrom or transmitted therefrom, respectively.
According to another aspect of the present invention, an image forming apparatus is provided with: an illumination light source for emitting an illumination light beam; a reflective type spatial light modulator for selectively reflecting the illumination light beam so as to modulate the illumination light beam into an image-carrying light beam; a light beam selecting surface for selectively reflecting the illumination light beam according to the angle of incidence at which the illumination light beam strikes the light beam selecting surface; and deflecting means disposed between the reflective type spatial light modulator and the light beam selecting surface so as to deflect at least one of the illumination light beam and the image-carrying light beam. Here, the illumination light beam is reflected from or transmitted through the light beam selecting surface so as to be shone, through the deflecting means, on the reflective type spatial light modulator, and is then selectively reflected by the reflective type spatial light modulator so as to be modulated into the image-carrying light beam, which is then shone, through the deflecting means again, on the light beam selecting surface so as to be transmitted therethrough or reflected therefrom according to whether the illumination light has been reflected therefrom or transmitted therefrom, respectively.